In a two gen system, two gen which were sharing equal load,one gen is disconnected and then after frequency for both reached 50hz ,gen is resynchronised in the network. so should they share same load as earlier or not?

Consider there are two turbines and two generators (say 1 and 2) in the system with two loads(load 1 and load 2) and the network is interconnected. And for governor and AVR, I am using controllers with set point for speed and voltage. I started up my turbine gen system (for both) and applied loads.

Both the generators are of same capactity and they shared the load equally. Now I disconnected one generator(gen 2) so as expected frequency for first gen drops and then its turbine power increased and first gen took up all the load(load 1 and 2). Meanwhile for gen two, after a sudden rise in frequency (at the moment of disconnecting this generator ), its frequency starts falling by valve closing and turbine power decreases for this gen 2. When both the generators reached 50 Hz frequency (where gen 1 is loaded and gen 2 is not loaded), I resynchronised gen 2 in the network. I observed a little drop in frequency and then turbine power increased a litttle more for both gen and now gen 2 does not share the same load as it shared earlier when both reached 50 hz(i.e. steady state.

Is the behaviour correct??

 

What type of control system are you running?

 

I am using normal controllers which have set points, lower and upper limits and can be set up to auto or manual mode, and work on basis of error.

 

Load sharing does not mean machines will equally provide/produce the same amount of power.

If you ever saw two machines configured to operate in Isoch speed control mode and parallel'd to each other, you would most likely experience a black-out very quickly. The two isoch machines cannot "share" load in any way shape or form (without some form of Isoch Load Sharing, which is really kind of a special kind of Droop speed control). The electrical load is usually "grabbed" by one machine and then the other machine "grabs" it back, and there are wild load swings, usually leading to a trip of one or both units.

So, now let's consider how we operate two machines that will be paralleled together driving a load which does not exceed the capability of either machine. Let's say the load is 15 MW and the two units are each rated at 20 MW. If one unit is supplying the load, it should be operated in Isoch speed control mode in order to maintain frequency. If the second unit is synchronized (paralleld) to the first, it should be in Droop speed control mode. When the breaker of the second unit closes, the second unit will usually "pick up" a little amount of power, let's say 1.5 MW. This means the Isoch unit's load will automatically be reduced to 13.5 MW. If the operator takes no action, the two units will continue to run like this indefinitely--"sharing" the load by not fighting each other to control the frequency. The Isoch unit is designated to be the frequency-control unit, and the Droop unit will only produce the load the operator desires, and changes with his governor Raise/Lower switch.

If the operator raises the load on the Droop unit to 7.5 MW, the load on the Isoch unit will drop to 7.5 MW. They will be sharing the load, equally in this case. If the operator increases the load on the Droop unit to 11.5 MW, the load on the Isoch unit will drop to 3.5 MW. They are still sharing the load, because the power production is stable and the frequency is stable.

Think about two small children wanting to play with the same toy. Neither one will let go of it and they both tug on it, sometimes until it gets broken! That's what it's like for two Isoch units trying to supply a load; they will fight over the load.

Load sharing means that all the units which are paralleled together to supply a common load will "play nice" together, and there will not be one or more units trying to grab more of the load than it should be.

Load sharing does not mean that two units connected together will produce the same amount of power. Period.

Now, having said all of the above, there are many kinds of control systems that can be employed to control multiple units supplying a load. These control systems can be programmed in any manner imaginable. Let's say your control system is/was set up to have the two generators produce an equal amount of load (I prefer *not* to say "share load"). If that's the case, then maybe there's some switch or button or permissive that must be pressed or active to drive the two units to split the load equally. Do you know? Can you find out?

But, from what you describe in your experiment, it seems everything is working as one would expect. What's not known is if there is an external load control system, and you haven't said how your units were configured (Isoch vs. Droop). And, again, control systems can be programmed/configured to behave in any way and do anything the programmer can make it do (which is sometimes very different from what the owners/operators had envisioned).

But, do not interpret load sharing to mean load splitting. 'Cause that's not what it means.

 

Thanks for your reply. I did not mean load splitting. Because I synchronised both machines which are of similar capacity together, and everything similar they shared load equally. I didn't understand your mail properly because I don't know much about droop and isochronous mode. Can you please explain that to me?

I think I should also give little introduction about my control system. For AVR and governor I am using controllers which have set points for voltage and speed respectively, a high and low limit ...and I am giving instantaneous speed and voltage as my inputs to controllers. Error is calculated at each instant, and turbine valves open/close based on that. So can I set isochronous or droop mode with this control system? if you can also guide me to the relevant literature it would be great. But anyways, firstly you teaching me things would be great. I have seen your response to steam turbine generator ...it was superb.

Thanks...

 

It would really help matters if you could explain what you mean by 'load sharing.' Because it's not clear what you mean if you don't mean (equal) load splitting. Load sharing means participating in providing the needed power to supply an electrical load, stably, and without causing any disturbance--regardless of the amount of load being produced or provided, or the percentage of the load being produced or provided. So, please, tell us what you mean by 'load sharing.'

On the left side of the control.com pages is a 'Search' field. You can type Isochronous or Droop in the field, click the 'search' button below (or just press Enter), and you will be blessed with much information about droop and isochronous speed control as it relates to GE heavy-duty gas turbine control. Actually, droop speed control is pretty much just that, except that instead of changing the speed reference some companies use a load reference. But, most utilities and governmental regulatory agencies require power producers to have some kind of droop speed control, usually set to a specific percentage, so that in the event of a grid frequency disturbance the machine will behave in such a way as to try to help the grid maintain nominal frequency. (Well, in most parts of the world, anyway. There are parts of the world that seem to perpetually run at less than rated or nominal frequency, and have pretty severe excursions, also. The Asian sub-contintent where this occurs shall remain nameless in this post.)

It's all been explained before. The whole point of control.com is to share information with as many people as possible; this isn't intended to be a personal training class, especially since this has been covered ad nauseum in the past.

Read through the postings you discover with the Search function. You'll find what you're looking for there.